In BP_Group_Extension, only fire the admin_screen_save() callback if it exists

This will avoid errors when the admin_screen() method is being used only to
display content, not to save settings.

See #4955

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6986 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Whitespace and coding standards in BP_Group_Extension::_register(). See #4955

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6987 cdf35c40-ae34-48e0-9cc9-0c9da1808c22